Arachidonic acid has attracted attention as a precursor of prostaglandins, thromboxane, prostacyclin, leucotrienes, etc. which have potent and various physiological actions including uterine muscle contraction, relaxation, vasodilatation, and antihypertensive action. Along with DHA (docosahexaenoic acid), it has extensively and intensively been investigated particularly as a substance essential for growth of infants. For example, Lanting et al. followed up the growth of infants until the age of 9 years who had been fed with mother's milk or milk powder for infants for more than 3 weeks after birth, investigated the incidence of minor impairments in the cranial nerve in these infants based on their behavior, etc, and found that the incidence of encephalopathy in the infants fed with milk powder for infants was about twice as high as that in those fed with mother's milk [LANCET, Vol.344, 1319-1322 (1994)]. This shocking fact is supposed to have been due to the lack of long-chain unsaturated fatty acids such as DHA and arachidonic acid in milk powder for infants while these acids are present in mother's milk, which acids may play an important role in development of the brain. Many studies have been done to make milk powder for infants resemble as closely as possible to mother's milk, the ideal nutrition for infants, though these studies have concentrated on elucidation of the relationship between the basic nutrients, vitamins, minerals, etc. present in mother's milk and the infection-preventing action of mother's milk. Lately the influence of long-chain poly-unsaturated fatty acids on the brain has also become of interest. Further, reports indicating that long-chain unsaturated fatty acids may play a role in development of the brain and the retina of newborns have recently been published one after another. This raises topics attracting attentions in the field of nutrition of premature infants and newborns. Thus it has been desired to develop oil containing arachidonic acid abundantly and being safely usable as ingredients of foods, in particular, infant formula.
Arachidonic acid occurs widely in the animal kingdoms, and has been isolated from lipids extracted from the adrenal gland and the liver of animals. However, because such organs contain the acid only a little and a large amount of the organs are hardly obtainable, isolation from these organs is insufficient for supply of arachidonic acid. Methods have been proposed to produce arachidonic acid by cultivation of various microorganisms capable of producing arachidonic acid. Among them those belonging to the genus Mortierella have been known to produce oil with a high content of arachidonic acid (Japanese Published Unexamined Patent Application No. 44891/88 and No. 12290/88). Although the oil thus produced are said to be highly safe, it is not widely accepted because of its originating in microorganisms. The oil obtained by cultivation of the microorganisms belonging to the species Mortierella alpine comprises mainly triglycerides (about 70% by weight or more) and phospholipids together with unsaponifiable matters including desmosterol. It is confirmed that sterol with cyclopropane structure which have never been known to occur in nature, in the concrete, 24,25-methylenecholest-5-en-3 .beta.-ol are contained among the unsaponifiable matters [LIPIDS, Vol.27, No.6, 481-483 (1992)], though all of the composition of the unsaponifiable matters in the oil is not known.